My nominations would be Basehart and Blackman in Dagger of the Mind.

Whilst the initial 'murder' is more an accident, they kill Wilfred Hyde White in cold blood, and Basehart is clearly losing his grip on reality by the end of the episode.

I agree strongly with Vivian Dmitri of "Rest in Peace, Mrs. Columbo" and Eric Mason of "How to Dial a Murder."

Also, let's not forget Justin Rowe of "Columbo Goes to College." His father drove him crazy.

Abigail Mitchell, while not really "psycho," is pretty scary when she says to Edmund, as she is slamming the safe on him, "Do you really think I didn't know?!"